The invention addresses a new type of production method for digital X-ray detectors, such as those used inter alia in medical diagnosis. As a rule, the size of these detectors is between 20×20 cm2 and 43×43 cm2. The current prior art is represented by detectors based on amorphous silicon (indirect conversion) and amorphous selenium (direct conversion). The principles of direct conversion (left) and indirect conversion (right) are shown in FIG. 1. With direct conversion I, an X-ray quantum 1 stimulates a particle 2, wherein electron/hole pairs 2a, 2b are generated and then migrate to the electrodes 4 (anode or cathode, for example pixel electrodes) where they are detected. With indirect conversion II, the X-ray quantum 1 stimulates the particle 2, which in turn emits radiation 2′ with low energy (for example visible light, UV or IR radiation), which is then detected by means of a photodetector 3 (for example a photodiode).
Indirect X-ray conversion includes the combination of a scintillator layer (for example Gd2O2S or CsI with different doping materials such as terbium, thallium, europium, etc.; layer thicknesses typically 0.1-1 mm) and a photodetector (preferably a photodiode). The emission wavelength of the scintillator light by means of X-ray conversion overlaps the spectral sensitivity of the photodetector.
In the case of direct X-ray conversion, the X-rays are, for example, again converted directly into electron/hole pairs, which are read out electronically (for example amorphous Se). Direct X-ray conversion into selenium is usually performed with layers with a thickness of up to 1 mm, which are pretensioned in the kV range in the blocking direction. While indirectly converting detectors have become established, in particular because they are simple and inexpensive to produce, direct converters have a much better resolving power.
One alternative to the aforementioned X-ray detectors based on inorganic semiconductors is hybrid-organic detectors, which to date are usually produced by application from the liquid phase. This in particular facilitates simple processing on large areas of up to 43×43 cm2 or more. The production of the detectors generally includes the introduction of the inorganic absorber materials such as, for example, typical scintillator materials into an organic matrix. Organic semiconductors can be easily applied to large areas from the liquid phase and the direct incorporation of the inorganic scintillator granules enables the optical cross talk to be significantly minimized.
Organic semiconductors have lower conductivity than inorganic semiconductors. This limited conductivity is problematic if, as for example with X-ray absorption, very thick layers are required to achieve sufficient sensitivity. This, on the one hand, reduces the efficiency of the photodiode since charge carrier extraction is impeded. On the other hand, the speed of the photodiode is reduced which limits usage for medical equipment, for example in the field of mammography in which only soft X-rays with a low penetration depth are used.
Organic semiconductors are primarily deposited from the liquid phase or in vacuum. All methods known to date for the incorporation of inorganic absorber materials use processing from the liquid phase.
U.S. Pat. No. 6,483,099 B1 describes the possibility of X-ray detection with a scintillator layer on an OPD (organic photodiode). Further embodiments include X-ray detection by the incorporation (“admixture”) of scintillators into an OPD, scintillators as a substrate or as part of the electrode. There is no information as how a scintillator can be incorporated homogeneously into a thick OPD layer or how, for example, to produce a 100 μm thick hybrid diode.
DE 101 37 012 A1 discloses an embodiment of a light-sensitive and polymer absorber layer with embedded scintillator granules. The conductivity of the polymeric layer is increased by the absorption of light from the scintillator. The mean distance of the scintillator granules in the layer corresponds to the mean free path length of the photons from the scintillator in the polymer.
DE 10 2010 043 749 A1 relates to an X-ray detector based on the above-described concept, wherein scintillators are either directly dispersed into the organic semiconductor solution or sprayed on in a “co-spraying process” at the same time as the organic semiconductor material.
The first case with liquid-phase application gives rise to the problem of creating a stable dispersion which is, in particular difficult with large scintillator particles. With small particles it is usual to add dispersing agents in order to prevent agglomeration of the particles, but this has a negative influence on the electrical properties of the organic semiconductors.
Both methods (liquid-phase application and vacuum deposition) have the drawback that, with the application of very thick layers (100 μm or more), enormous quantities of solvents have to be released and the layers are very rough. Complete evaporation of the solvents is not only a technical requirement, it also represents a health and critical environmental problem.
Hence, there is a requirement for the production of X-ray detectors based on inorganic absorber materials, such as typical scintillator materials, which are incorporated into an organic semiconductor matrix. This combination should have the advantages of combining the two aforementioned concepts with one another. Organic semiconductors are easy to apply to large areas from the liquid phase and the direct incorporation of the inorganic scintillator granules enables optical crosstalk to be significantly minimized. The main problem with these hybrid-organic photodetectors is the processing of thick layers. The material suggested here enables the production of thick layers.